An ad hoc hypothesis thrown up by Christians when confronted with intractable contradiction in their absurd belief system. It is nothing more than a veiled confession of ignorance from someone who will say anything rather than question or confront their beliefs. When arguing with a believer, hearing this usually means you've won the debate.

"How can an all-loving god condemn so many people to eternal torment in hell?"
"Simple, god gave us a choice to either accept him or reject him."
"But if God cares so much about us, then why does he need to coerce our devotion with the threat of torture? So why doesn't God reveal himself and warn us about it? Why doesn't he set the non-believers straight? Is our having faith so important to him that he's willing to let millions of people be condemned?"
"Uh...God works in mysterious ways!"